Joint Biomechanics

Levers & Loads - Forceful Fun

• Levers: Rigid bars; Fulcrum (F), Effort (E), Resistance (R).
• 📌 FRE 123:
  • Class 1: F in middle (Triceps/Elbow extension). MA varies.
  • Class 2: R in middle (Calf raise). MA > 1.
  • Class 3: E in middle (Biceps/Elbow flexion). MA < 1; ↑ROM/speed.
• Loads: Forces on joints. Types: Compressive, Tensile, Shear.
• Stress ($σ$) = $F/A$.
• Strain ($ε$) = $ΔL/L_0$.

⭐ The most common type of lever in the human body is Class 3.

Joint Junctions - Motion Masters

• Joint Classification:
  • Synarthrosis: Immovable (e.g., skull sutures).
  • Amphiarthrosis: Slightly movable (e.g., pubic symphysis, intervertebral discs).
  • Diarthrosis (Synovial): Freely movable. Key features: articular (hyaline) cartilage, joint capsule, synovial membrane & fluid.
• Synovial Joint Types & Axes:
  • Plane (non-axial), Hinge (uniaxial), Pivot (uniaxial), Condyloid (biaxial), Saddle (biaxial), Ball-and-socket (multiaxial).
• Joint Kinematics:
  • Osteokinematics: Physiological motion of bones (e.g., flexion, extension, abduction).
  • Arthrokinematics: Accessory motions at joint surfaces.
    • Roll, Slide (Glide), Spin.

⭐ Convex-Concave Rule: Convex on concave: roll & slide opposite. Concave on convex: roll & slide same direction.

Cartilage & Cushions - Smooth Operators

Articular (hyaline) cartilage: smooth, low-friction, shock-absorbing surface in synovial joints.

• Composition:
  • Water: 70-80% by weight.
  • Cells: Chondrocytes (maintain ECM).
  • ECM: Type II collagen (tensile strength), Aggrecan (compressive stiffness, binds water).
• Structure (Zones): 📌 Superficial Tangential Deep Calcified (STDC).
  • Superficial: Collagen parallel, resists shear.
  • Transitional: Random collagen.
  • Deep: Collagen perpendicular, resists compression. Tidemark separates from calcified zone.
• Properties: Avascular, aneural, alymphatic (limited repair). Nutrition via synovial fluid. Low friction coefficient (e.g., 0.002-0.02). Viscoelastic (creep, stress relaxation).
• Lubrication Mechanisms:
  • Boundary: Lubricin (PRG4) coats surfaces.
  • Fluid Film: Weeping, elastohydrodynamic, boosted.

⭐ Aggrecan is the major proteoglycan in articular cartilage, responsible for its compressive stiffness.

Soft Tissue Supports - Elastic Enablers

• Ligaments & Tendons: Viscoelastic; vital for joint stability & motion.
  • 📌 Mnemonic: Ligaments Bind Bones (LBB); Tendons Marry Muscles to Bones (TMB).
  • Composition: Primarily Type I collagen, elastin.

    ⭐ Ligaments and tendons are primarily composed of Type I collagen.

  • Biomechanics: Show creep (deformation at constant load) & stress relaxation (stress ↓ at constant deformation).
  • Stress-Strain Curve:
    • Toe region: Collagen crimp straightens.
    • Linear region: Elastic deformation.
    • Failure region: Microscopic to macroscopic tears.
  • Ligaments: Bone-to-bone; stability. Failure strain 20-25%.
  • Tendons: Muscle-to-bone; transmit force. Stronger, less extensible. Failure strain 8-15%.
• Synovial Fluid:
  • Functions: Lubrication (hyaluronan, lubricin), nutrition, shock absorption.
  • Nature: Non-Newtonian (viscosity ↓ with ↑ shear rate).

High‑Yield Points - ⚡ Biggest Takeaways

• Stress-Strain Curve defines material behavior; Wolff's Law dictates bone remodeling under stress.
• Synovial joints have ultra-low friction due to hyaline cartilage and synovial fluid.
• Musculoskeletal system mainly uses Class 3 levers, favoring speed/range over force.
• Viscoelasticity (creep, stress relaxation) is vital for soft tissue function (e.g., tendons, ligaments).
• Joint Reaction Force (JRF) is the net force acting across a joint during activity.
• Normal gait cycle consists of 60% stance phase and 40% swing phase.